A conventional process of preparing organic silane compounds is to cause hydrogenated silane containing compounds to react with unsaturated organic compounds in the presence of a catalyst, which is generally referred to as a hydrosilylation reaction. The hydrosilylation reaction is commonly carried out by successively adding hydro alkoxy silanes to unsaturated group containing organic compounds. A chloroplatinic acid catalyst (H2PtCl6 6H2O) as a catalyst for the hydrosilylation reaction is disclosed in Japanese Patent Laid-open Nos. (Sho) 63-250390, (Sho). 63-313793, and (Hei) 11-323132. However, in a case where the platinum chloride catalyst is used in the hydrosilylation reaction, there are problems in that it is difficult to remove the heat significantly generated during the initial reaction, and in a batch reaction system, each reaction is not constantly processed. Further, when a reaction temperature is increased so as to improve the reaction yield, the problem may arise that by-products are considerably generated.